Vehicle emission regulations have become increasingly stringent, according to recently strengthened environmental and safety regulations. That is, in order to cope with the requirement to be lightweight yet provide improvements in collision safety for improving fuel economy, applications for high-strength steels including, for example, an advanced high-strength steel (AHSS), have increased.
In particular, applications for ultra high-strength steels having a strength of 1000 MPa or more are inevitable, and various methods for the formation thereof have been researched and developed.
As shown in FIG. 1, since elongation becomes very low instead of securing high tensile strength with respect to ultra high-strength steels, there exist many limitations in the formation thereof.
A hot press forming (referred, to simply as ‘HPF’) technique was developed as a method for resolving the foregoing limitations, and the HPF technique is a technique of manufacturing parts using press hardening characteristics.
The HPF technique is a new sheet forming method, in which a sheet of a material having high hardenability, such as a boron steel, is heated to a high temperature, and then formed by using a die at room temperature. The HPF technique has been applied to dozens of automotive parts, focusing on European and American automobiles, after the technique was developed by a Swedish steel maker, SSAB plannja AB, in 1973. Recently, the applications thereof have also been increased in South Korea.
The HPF process is a processing method, in which a steel having improved hardenability by adding elements with high hardenability, such as boron (B), molybdenum (Mo), or chromium (Cr), is heated above an Ac3 transformation point, a high temperature of about 900° C., and a product is then immediately hot formed in a press die and rapidly cooled to manufacture a high-strength product.
FIG. 2 schematically illustrates a HPF process.
The HPF process may be categorized as a direct method and an indirect method, and each method is briefly illustrated in FIG. 3.
As shown in FIG. 3, the direct method is a method of simultaneously performing press forming and die quenching at high temperatures, and the indirect method is a method of die quenching by heating at high temperatures after partially or completely forming a part at room temperature.
The advantages and disadvantages of each method are described below.
1) The direct method has an advantage in that the process thereof is simple, because forming and quenching are performed in a die set at the same time, but has a disadvantage that there are limitations in manufacturing drawing type parts, because friction characteristics are very poor at high temperatures.
2) The indirect method has disadvantages that the process thereof must be divided into two because press forming must first be performed at room temperature and as a result, processing costs increase in comparison to the direct method, but has an advantage that the manufacturing of drawing type complex parts is possible because the direct method is a room temperature forming method.
Meanwhile, parts applied for a crash member may largely be categorized into two types.
First, an energy absorption part is a part that absorbs impacts applied from the outside through deformation.
Typically, a front side of a front side member, a rear side of a rear side member, and a lower side of a B-pillar correspond to energy absorption parts.
Second, an anti-intrusion part is a part in which deformation is almost not generated. For example, since a cabin zone including passengers needs to be secured during crash, crash members applied thereto mostly correspond to anti-intrusion parts.
Typically, the anti-intrusion part may include a rear side of the front side member, a front side of the rear side member, and an upper side of the B-pillar. Therefore, cases of improving crashworthiness by applying HPF are rapidly increased with respect to the anti-intrusion part, and AHSS having relatively high elongation has been applied to the energy absorption part.
As described above, members, such as the front side member, the rear side member, and the B-pillar, have a form in which an energy absorption part and an anti-intrusion part are combined with each other, and have generally been used by respectively forming two parts and welding them together.
In order to resolve the foregoing limitation of the respective forming of the two parts, a method of applying HPF steel and general high-strength steel by making a tailor welded blank (TWB) and a method of obtaining different strengths in a single part by differing heat treatment characteristics for sections have been suggested.
In particular, the method of obtaining differences in strengths by differing heat treatment characteristics is largely divided by cooling rate control and heating temperature control methods.
The heating temperature control method is a method of controlling phase transformation by differing heating temperatures in a high-strength region and a high-elongation region, and has an advantage that maintaining a short cycle time is possible, but has a disadvantage that an additional heating device may be necessary.
Meanwhile, the cooling control method includes a method of controlling a cooling rate by setting a die temperature of a high-elongation region to be high and a method of controlling a contact area by setting a gap or a groove of the high-elongation region to be large. The former has an advantage in that the realization thereof may be easy, but has disadvantages that a device for controlling the die temperature may be necessary and a cycle time may increase, and the latter has disadvantages in that processing may be necessary for a complex die, and a cycle time may increase, although the method is conceptually possible.
An aspect of the present invention provides a method of manufacturing a multi physical properties part, in which the multi physical properties part may be more economically and simply manufactured by using two or more separated die sets, without using an additional heating device or treating a die surface.